Improved "constraints" claim description per feedback from Nat

[selfissued]

As Nat wrote:

• constraints (Section 13.3) is defined as constraints pertaining to the JWT with application-specific details, and the example states, "For instance, the constraints Claim might be used to impose material thickness limits for a physical object." It is not a very intuitive example. Explaining what is written in Section 6.2 would have been clearer.

This PR removes the unintuitive "material thickness" example, instead using the example of constraints on Trust Chains as used in the specification, as suggested by @sakimura.